Conventionally, when mounting various automotive component parts on a panel member of an automobile, push rivets are extensively used. A push rivet consists of two parts. The first part consists of a grommet part including a leg portion adapted to be passed into a mounting hole of a panel member and a flange integrally provided at its upper end, with a central longitudinal bore passed entirely through the grommet part. The second part consists of a pin part including a pin head, and a pin shank depending therefrom. The pin shank includes a part having an outer diameter greater than the inner diameter of the central bore of the leg portion of the grommet part. As a result, when the leg portion of the grommet part is passed into a mounting hole of a panel member, and the shank of the pin part is forced into the central bore of the grommet part, the leg portion of the grommet part is forced to expand radially, and becomes tightly and securely engaged with the mounting hole.
Japanese utility model laid-open (kokai) publication No. 1-165810 discloses a push rivet which can be easily unfastened when necessary. This push rivet includes a cam structure which causes the pin part to recede out of the grommet part and the leg portion to be radially contracted when the head of the pin part is turned by using a suitable tool such as a screwdriver so that the push rivet may be unfastened without damaging it, and it may be used again.
However, according to this proposal, when an attempt is made to turn the head with the intention to dislodge the pin part, the grommet part tends to turn with the pin part, thereby preventing the receding movement of the pin part. It is therefore often necessary to secure the grommet part before turning the head and unfasten the push rivet. It is highly inconvenient to say the least, and sometimes impractical because the push rivet may not be located in a readily accessible part. Furthermore, depending on the location of the push rivet, even engaging a tool with the head of the pin part may cause a great difficulty, let alone turning it.